Re: [RFC PATCH 0/3] Change how we determine when to hand out THPs

[Andy Lutomirski]

On Tue, Dec 17, 2013 at 8:04 AM, Alex Thorlton <athorlton@xxxxxxx> wrote:
> On Mon, Dec 16, 2013 at 05:43:40PM -0800, Andy Lutomirski wrote:
>> On Mon, Dec 16, 2013 at 9:12 AM, Alex Thorlton <athorlton@xxxxxxx> wrote:
>> >> Please cc Andrea on this.
>> >
>> > I'm going to clean up a few small things for a v2 pretty soon, I'll be
>> > sure to cc Andrea there.
>> >
>> >> > My proposed solution to the problem is to allow users to set a
>> >> > threshold at which THPs will be handed out.  The idea here is that, when
>> >> > a user faults in a page in an area where they would usually be handed a
>> >> > THP, we pull 512 pages off the free list, as we would with a regular
>> >> > THP, but we only fault in single pages from that chunk, until the user
>> >> > has faulted in enough pages to pass the threshold we've set.  Once they
>> >> > pass the threshold, we do the necessary work to turn our 512 page chunk
>> >> > into a proper THP.  As it stands now, if the user tries to fault in
>> >> > pages from different nodes, we completely give up on ever turning a
>> >> > particular chunk into a THP, and just fault in the 4K pages as they're
>> >> > requested.  We may want to make this tunable in the future (i.e. allow
>> >> > them to fault in from only 2 different nodes).
>> >>
>> >> OK.  But all 512 pages reside on the same node, yes?  Whereas with thp
>> >> disabled those 512 pages would have resided closer to the CPUs which
>> >> instantiated them.
>> >
>> > As it stands right now, yes, since we're pulling a 512 page contiguous
>> > chunk off the free list, everything from that chunk will reside on the
>> > same node, but as I (stupidly) forgot to mention in my original e-mail,
>> > one piece I have yet to add is the functionality to put the remaining
>> > unfaulted pages from our chunk *back* on the free list after we give up
>> > on handing out a THP.  Once this is in there, things will behave more
>> > like they do when THP is turned completely off, i.e. pages will get
>> > faulted in closer to the CPU that first referenced them once we give up
>> > on handing out the THP.
>>
>> This sounds like it's almost the worst possible behavior wrt avoiding
>> memory fragmentation.  If userspace mmaps a very large region and then
>> starts accessing it randomly, it will allocate a bunch of contiguous
>> 512-page regions, claim one page from each, and return the other 511
>> pages to the free list.  Memory is now maximally fragmented from the
>> point of view of future THP allocations.
>
> Maybe I'm missing the point here to some degree, but the way I think
> about this is that if we trigger the behavior to return the pages to the
> free list, we don't *want* future THP allocations in that range of
> memory for the current process anyways.  So, having the memory be
> fragmented from the point of view of future THP allocations isn't an
> issue.
>

Except that you're causing a problem for the whole system because one
process is triggering the "hugepages aren't helpful" heuristic.

--Andy

> - Alex



-- 
Andy Lutomirski
AMA Capital Management, LLC
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